Device for compensating the astigmatism of electron lenses



1958 A. c. VAN DORSTEN 2,862,129 v DEVICE FOR COMPENSATING THE AST IGMATISM 0F ELECTRON LENSES Filed-Feb. 28. 1955 INVENTOR ADRIANUS CORNELIS VAN DORSTEN AGE T DEVICE FOR COMPENSATIN G THE ASTIGMA- TISM F ELEiZTRON LENSES Adrianus Cornelis van Dorsten, Emmasingel, Eindhoven, Netherlands, assignor, by mesne assignments, to North American Philips Company, Inc., New York, N. Y., a corporation of Delaware Application February 28, 1955, Serial No. 491,161 Claims priority, application Netherlands March 11, 1954 4 Claims. (Cl. 313-66) For the concentration of rapidly moving, electrically charged particles use is made of magnetic or electric electron lenses. With the aid of rotational-symmetrical lens fields the electrically charged particles are concentrated in a dot-shaped sectional area. Other fields having no rotational symmetry may be provided to restrict the lateral deviation of charged particles from a preferential direction. The last-mentioned fields may also be used to correct particular errors of the rotational-symmetrical lenses. Their eifect is comparatively small and restricted to small deflections of the course of the charged particles.

The auxiliary fields permit of correcting the astigatism of a lens, i. e. an error of the rotational symmetry.

It is due to astigmatism that only an approximately dotshaped image can be made of a dot-shaped source of rays. The imperfection of the resultant image is due to disturbance of the rotational symmetry of the lens-field. Inaccuracies in machining and contaminations of the pole shoes and other members, for example limiting diaphragms, arranged in the proximity of the course of rays, have inevitably an adverse effect, which becomes manifest already after a short time of operation. These contaminations give rise to irregular electrical charges which disturb the rotational symmetry.

Devices are known by which an auxiliary field for correcting this astigmatism is produced. They comprise a plurality of auxiliary electrodes or auxiliary poles lying in a plane at right angles to the axis of the beam and symmetrically around this axis. The intensity of the auxiliary field is controlled by varying the potentials of the auxiliary electrodes or the energization of the auxiliary poles. The number of electrodes or auxiliary poles is a multiple of two, so that a correcting astimgatism can be produced which can assume all possible values.

In a further known device a correcting auxiliary field is produced by means of a cylindrical insert in one of the pole shoes of a magnetic electron lens, this insert embracing the electron rays. The correction efiect is obtained in that along the edge of the end facing the pole surface of the electron lens the insert has a height varying throughout the periphery in a manner such that there is symmetry relative to two planes through the axis at right angles to one another. In order to adjust the intensity and the direction of the deformation of the lens field, the insert must be displaceable in an axial direction and be rotatable about its axis.

The invention relates to a device for compensating astigmatism of the mainly rotational-symmetrical field of a lens for corpuscular rays by means of a separate, nonradial symmetrical auxiliary field, this device having the advantage that the correction means may be arranged at any desired small distance from the axis of the lens. According to the invention the device comprises at least two cylindrical correction members surrounding the axis of symmetry of the lens and having a height varying throughout the periphery and having two planes of symmetry at right angles to one another, intersecting one another in the axis, provision being made of means for nited States Patent Face producing and electrically controlling the auxiliary field determined geometrically by the correction members. The correction of a magnetic lens "by means of this device requires two cylindrical inserts, which are arranged one behind the other and which are provided each with an energizing winding. At least one edge of the two cylinders has a profile of elevations and depressions, which are symmetrical relative to two planes through the axis at right angles to one another, the corresponding planes of symmetry of which are, however, turned relatively to one another. At the energization of the two windings, each of the cylinders produces a magnetic field which unites the properties of a rotational-symmetrical lens and of a so-called cylinder lens. The auxiliary .fields thus obtained by two inserts exhibit deviations from the rotational symmetry, which have a relative shift of 90". By controlling the intensity of each field, the resultant astigmatic disturbance may be turned through and its intensity may be adjusted at will.

By means of three insert cylinders, of which the edges facing one another are profiled, one edge being the reflection of the other, and of which the planes'of symmetry have a shift of 90, a correcting, electric auxiliary field may be produced by applying a suitable potential to each insert.

By means of two cylindrical inserts, of which the edges facing one another are profiled, the desired correction may be obtained by using a combination of an electrostatic field and a magnetic field, the intensities of which are variable.

The various possibilities will be explained with reference to the drawing, in which Figs. 1 and 2 show part of an electron-microscope comprising correction means according to the invention and 'Fig. 3 shows one embodiment of a combination of inserts for electric and magnetic control.

The magnetic electron lens shown in Fig. 1 may be the objective of an electron microscope. It comprises two pole shoes 1 and 2 of magnetic material. The pole surfaces are spaced apart by a small distance 3. By minimizing the bores 4 and 5, the spherical and chromatic aberration of the lens may be restricted to very small values. The mechanical working of these parts is never without inaccuracies, which give rise to deviations from the rotational symmetry of the lens field; A further cause of such a deviation resides in the inhomogeneity of the magnetisable material of which the pole shoes are made.

In order to keep the pole shoes spaced part, they are interconnected by a ring of non-magnetisable material 6, for example brass.

Reference numeral 7 designates the yoke connecting the pole shoes, in which yoke the energizing winding 8, arranged on the coil former 9, produces a magnetic flux.

Reference numerals 10 and 11 designate parts of the wall of the microscope adjacent the lens.

The electrons travel through the vacuum space from top to bottom and arrive, subsequent to their passage through the lens, at a correction device. This comprises two inserts 12 and 13 of magnetisable material. are constituted by tubular bodies, of which the edges at the ends facing the pole surfaces have a height varying;

throughout the periphery. Figure 1 shows one embodiment, of which the edge is corrugated. As an alternative,. 7

it may, however, have a toothed profile or exhibit recaxis at right angles to one another.

must have a shift of 90.

They" inder 13. They are both arranged within the prolonga-v tion of a part of the pole shoe 2 of magnetisable mate rial in order that the flux of the auxiliary field to correct the astigmatism of the objectivelens should passmainly through such'metal. For the energization of-the insert cylinders use is" made of the magnetizing windings :14 and 15, of which the supply wires 16, 17 and 18 .are taken through the wall of the microscope to the outside. With the aid of only three wires and the otentiometers 19 and 20, connected to the supply 21, the energization or" each coil may be adjusted separately and the direction and the intensity of the correcting field may be controlled. The astigmatism of the lens may be corrected by the artificial astigmatism thus introduced and by such acontrol of theenergizationof each insert cylinder that the artificial deviation is of the same order as the deviation to be corrected. The correction need not take place in the proximity of the lens field. It may be carried out at the area where the arrangementgives rise to the least difficulties;

Fig. 2 shows an electrostatic lens comprising two outer electrodes 22 and 23, having the same potentials and being connected to earth. The two electrodes have central apertures 24 and 25 respectively. Between the two outer electrodes is arranged an electrode 26 having a high potential and an aperture 27. This electrode is supported from the insulator 28. For the correction of the astigmatism used is made of the insert cylinders 29, 30 and 31 of which the edges facing one another have the profile referred to above. The two edges of the cylinder 30 are curved. This cylinder is connected electrically to the wall 32 of the microscope and is thus at earth potential. The two further cylinders 29 and 31 have voltage supply wires 33 and 34, which are taken in an insulated manner through the wall 32. By means of the potentiometers 35 and 36, which are connected to the voltage supply 37, the potential of the cylinders 29 and 31 may be adjusted separately. The electrostatic fields between the cylinders 29 and 30 and between the cylinders 30 and 31 exhibit, as far as the direction and the intensity are'concerned, each a particular'astigmatism, of which the resultant effect on the electron rays can be controlled in a manner such that the deviation to by the combination of a magnetic and an electrostatic i auxiliary field. To this end use is made of two cylindrical inserts 38 and 39, as shown in Fig. 3, of which the edges facing one another exhibit a reflectional-symmetrical profile. The energizing winding 40 is common to both of them and provides the magnetization required for producing the magnetic auxiliary field. Between the two cylinders an electrical potential difference is maintained in order to produce the electrostatic auxiliary field. The deviation resulting from the magnetization has a shift of owing to the different ways in which an electron ray is acted upon by a magnetic field and an electric field, relative to the deviation produced by the electrostatic field with the shapes of the electrode edges shown, so that by controlling the field strength also a resultant astigmatism is obtained, which may assume all possible values.

Not only for electron microscopes, but in any kind of devices for rapid charge carriers, in which the concentration of the particles and, if desired, the. production of perfect images by these particles can be furthered by the introduction of the artificial compensating astigmatism, use may be made of the correction means shown.

What is claimed is:

1. An electron lens system comprising .in combination, means for concentrating rapidly-moving charged particles into a beam having a dot-shaped cross-sectional area, said concentrating means including means to produce a field of substantially rotational symmetry about an axis thereof, and means to compensate for the astigmatism of the field produced by said field producing means, said astigmatism compensating means being disposed at a position at which the charged particles arrive subsequent to their passage through the field and comprising a pair of tubular members spaced apart and arranged along the axis, said members having two spaced peripheral edges thereof which vary in height symmetrically in two perpendicular planes intersecting said axis at right angles,

and means, for producing a field within said members determined by the geometry thereof.

2. A device as claimed in claim 1 in which the tubular members are composed of magnetic material and are provided each with a separate energizing winding.

3. A device as claimed in claim 1 in which three correcting bodies are arranged one behind the other, the outer bodies having potential differences relative to the intermediate body.

4. A device as claimed in claim 1 in which two-'correcting bodies. are arranged one behind the other and have a common energizing winding with a potential difference between the two bodies.

References Cited in the file of this patent V UNITED STATES PATENTS 2,313,018 

